1. Field of the Invention
The present invention relates to proton accelerators.
2. Description of Related Art
In the late 1920s and early 1930s, research in experimental nuclear physics was stimulated by the invention of several types of particle accelerators. These systems included the radio frequency (RF) drift-tube linear accelerator by Rolf Wideröe, the RF spiral-orbit cyclotron by Ernest Lawrence, the direct current (dc) cascaded-rectifier high-voltage generator by John Cockcroft and Ernest Walton, and the dc electrostatic high-voltage generator by Robert Van de Graaff. Approximately 600 Van de Graaff ion and electron accelerators were made by the High Voltage Engineering Corporation, which was founded in 1946 by several professors from the Massachusetts Institute of Technology (MIT). Those electrostatic systems were popular because of their ability to provide small-diameter, low-divergence particle beams with finely controlled energies. The ion sources were typically small, glass tubes containing plasmas excited by low-power RF generators. The proton beam current was limited to a few hundred microamperes, but this was usually sufficient for many research programs in nuclear physics.
Physicists and other scientists sought out accelerators that could provide higher beam currents for a variety of applications. For example, the U.S. National Aeronautics and Space Administration (NASA) sought accelerators that could provide higher proton beam currents to investigate the deleterious effects of the Van Allen radiation on satellites in space. Their need motivated the development of Dynamitron dc accelerators with Duoplasmatron type ion sources (M. von Ardenne, Tabellen der Electrophysik, Ionenphysik und Ubermikroskopie I, V.E.B. Deutcher Verlag der Wissenschaften, 544-549 (1956); C. D. Moak, H. E. Banta, J. N. Thurston, J. W. Johnson, R. F. King, Duoplasmatron Ion Source for Use in Accelerators, Rev. Sci. Instrum. 30, 694 (1959)). The modified Duoplasmatron ion sources developed by Radiation Dynamics, Inc. (RDI) were capable of emitting more than 10 mA of atomic, diatomic and triatomic ions obtained from hydrogen or deuterium plasmas (M. R. Cleland, R. A. Kiesling, Dynamag Ion Source with Open Cylindrical Extractor, IEEE Transactions on Nuclear Science, NS-14, No. 3, 60-64 (1967); M. R. Cleland, C. C. Thompson, Jr., Positive Ion Source for Use with a Duoplasmatron, U.S. Pat. No. 3,458,743, Patented Jul. 29, 1969.). (Recently, RDI's name has been changed to IBA Industrial, Inc.)
For another example, the fast-neutron cancer therapy system that was developed during the early 1970s by RDI, in cooperation with AEG Telefunken for the University Hospital Hamburg-Eppendorf in Germany, accelerated a 12 mA beam of atomic and molecular deuterium ions to an energy of 600 keV to produce a high-intensity source of 14 MeV neutrons (>2×1012 neutrons per second) from a rotating, tritium-coated target (M. R. Cleland, The Dynagen IV Fast Neutron Therapy System, Proceedings of the Work-Shop on Practical Clinical Criteria for a Fast Neutron Generator, Tufts-New England Medical Center, Boston, Mass., 178-189 (1973) and B. P. Offermann, Neutron-Therapy Unit for the Universitätskrankenhaus Hamburg-Eppendorf Radiologische Universitätsklinik, in the same Work-Shop Proceedings, 67-86 (1973).
However, the acceleration of a mixed beam of atomic and molecular hydrogen ions to higher energies (up to 4.5 MeV) in larger Dynamitrons was limited to only a few milliamperes. The collisions of energetic ions with residual hydrogen gas from the ion source, which was flowing through the longer acceleration tube, had the undesirable affect of producing unfocussed hydrogen ions and free electrons. Some of these unwanted ions and electrons were intercepted by intermediate dynodes, which distorted the voltage distribution along the acceleration tube. This effect led to unstable operation at higher beam currents. The free electrons produced by these collisions were drawn back toward the positive high-voltage terminal, where they generated X-rays. The X-rays produced ions in the high-pressure sulfur hexafluoride gas that was used to insulate the high-voltage generator. This effect was indicated by the dc current flowing from the high-voltage rectifier column to the RF electrodes which surrounded and energized the cascaded rectifier system, and it was verified by measuring the X-ray pattern outside of the pressure vessel. The generation of X-rays by free electrons within the acceleration tube was undesirable because it wasted high-voltage power and increased the radiation shielding requirements in the accelerator facility.
Further studies demonstrated that the ion current limitations described above could be alleviated by adding a titanium getter pump near the ion source to reduce the flow of hydrogen gas into the acceleration tube. An electrostatic einzel lens and a crossed electric and magnetic field mass analyzer were also added after the ion source to deflect the molecular hydrogen ions and prevent them from entering the acceleration tube (E. M. Kellogg, Ion-Gas Collisions During Beam Acceleration, IEEE Transactions on Nuclear Science, Vol. NS-12, No. 3, 242-246 (1965); M. R. Cleland, P. R. Hanley, C. C. Thompson, Acceleration of Intense Positive Ion Beams at Megavolt Potentials, IEEE Transactions on Nuclear Science, Vol. NS-16, No. 3, 113-116 (1969)).
However, high-energy dc proton accelerators, capable of providing more beam current than a few milliamperes, have not been developed previously. There are a number of very important applications that require or could benefit from a high-current, high-energy dc proton accelerator. For example, applications such as boron neutron capture therapy (BNCT), the detection of explosive materials by nuclear resonance absorption (NRA) and the cleavage of silica for the production of thin silicon wafers, such as those used for solar cells, would benefit from an accelerator with such capabilities.
Despite the growing need for such an accelerator, previous attempts to develop a proton accelerator, with both high-current and high-power capabilities, have not been successful. A high-current, high-energy pulsed proton beam could be produced by using a radio-frequency quadrupole (RFQ) accelerator. Nevertheless, a dc proton accelerator would be more desirable because it is more efficient electrically, and it can produce a continuous beam, in contrast to the pulsed beam from an RFQ accelerator. A continuous dc beam can produce a more uniform dose distribution than a pulsed beam when it is scanned over a large area target. A dc accelerator can also produce a proton beam with less energy variation, which is important for NRA applications and for the production of thin silicon wafers.